1. Technical Field
The present invention generally relates to a refrigerant compressor, and more particularly, to a slant plate type compressor, such as a wobble plate type compressor, with a variable displacement mechanism which is suitable for use in an automotive air conditioning system.
2. Description Of The Prior Art
A wobble plate type compressor with a variable displacement mechanism suitable for use in an automotive air conditioning system is disclosed in Japanese Utility Model Application Publication No. 64-27487. The compressor is driven by a the engine of the automobile.
The above compressor includes a variable displacement mechanism which comprises a first communication path linking a crank chamber and a suction chamber in fluid communication, and a second communication path linking the crank chamber and a discharge chamber. A first valve control mechanism controlling the opening and closing of the first communication path is disposed within the first communication path. A second valve control mechanism controlling the opening and closing of the second communication path is disposed within the second communication path. The first communication path is provided with a first valve seat formed at one portion thereof. The second communication path is provided with a second valve seat formed at one portion thereof. The first valve control mechanism includes a first valve member which is disposed so as to be received on and moved away from the first valve seat. The second valve control mechanism includes a second valve member which is disposed so as to be received on and moved away from the second valve seat.
The first and second valve members are linked through a rod member so that when the first valve member is received on the first valve seat to close the first communication path, the second valve member is moved away from the second valve seat to open the second communication path. Conversely, when the first valve member is moved away from the first valve seat, the second valve member is received on the second valve seat.
During operation of the compressor, the capacity of the compressor depends upon the crank chamber pressure relative to the suction chamber pressure, with the compressor operating at maximum capacity when the crank and suction chambers are linked in fluid communication. When the link between the crank and suction chambers is terminated, simultaneously linking the crank and discharge chambers, the pressure in the crank chamber increases relative to the suction chamber due to the flow of high pressure fluid from the discharge chamber to the crank chamber, thereby reducing capacity. Of course, when operating at reduced capacity, the power demands of the compressor on the automobile engine are reduced as well.
The first valve control mechanism includes a pressure sensing device such as a diaphragm, one side of which senses the pressure in the suction chamber. The opposite side of the diaphragm is acted upon by a cylindrical member made of magnetic material and forming part of a solenoid mechanism. The relative position of the cylindrical member, and thus the effective force provided thereby upon the diaphragm, is controlled by the solenoid in response to an external vehicle condition, such as the power demands made upon the engine of the vehicle.
The diaphragm is responsive to the net force acting on the opposite sides thereof and acts upon the rod member linking the first and second valve members to simultaneously control the opening and closing of the two communication paths. For a given positioning of the cylindrical member, the effect thereof on the diaphragm is constant, and the diaphragm responds to changes in the suction pressure to act upon the rod member to control the link between the crank and suction chambers. Thus, for a given positioning of the cylindrical member, the first valve member acts to maintain the suction pressure at a predetermined constant value. By changing the position of the cylindrical member through operation of the solenoid in response to the demands made upon the engine of the vehicle, the predetermined constant value of the suction pressure can be changed in response to the demands made upon the engine.
As discussed above, the compressor operates at maximum capacity when the crank and suction chambers are linked. This linkage occurs when the suction pressure exceeds the predetermined constant value and acts upon the diaphragm to move the first valve member away from the first valve seat, simultaneously isolating the crank and discharge chambers. For example, when the heat load on the evaporator of the automotive air conditioning system is great, the suction pressure will be great, causing the crank and suction chambers to be linked, maximizing capacity.
However, when the first valve member acts to maintain the suction pressure at the predetermined constant value for the given positioning of the cylindrical member, the second valve member which is linked to the first valve member through the rod member continuously receives the discharge pressure. The value of the discharge pressure is varied by unexpected changes in heat exchanging capability of a condenser of the automotive air conditioning system caused by such conditions as changes in velocity of the automobile. Accordingly, the force acting downwardly on the rod member can be unexpectedly varied in response to changes in the discharge pressure so that an undesirable change in the predetermined constant value in the suction chamber can occur even though an electric current at a constant amperage is supplied to the solenoid to provide a constant electromagnetic force. Thus, in this prior art compressor, the suction pressure cannot be stably maintained at a predetermined constant value by controlling communication between the crank and suction chambers.
Furthermore, in the above prior art compressor, when power demand on the vehicle is great, it is not desirable for the compressor to operate at maximum capacity, even if the heat load on the evaporator and the corresponding suction pressure are large. The solenoid is responsive to the greater demand for power to increase the effect of the cylindrical member upon the diaphragm, for example, by reducing the force with which the cylindrical member is pulled away from the diaphragm. Thus, the predetermined constant value at which the suction pressure is maintained will be increased, requiring an even greater pressure in the suction chamber before the crank and suction chambers can be linked.
For example, even if suction pressure is increased due to an increase of the heat load on the evaporator, the compressor will not function at maximum capacity while the demand for engine power by the vehicle is large, since the crank and suction chambers will be isolated. Correspondingly, the crank and discharge chambers will be linked, rapidly increasing the crank pressure relative to the suction pressure to minimize compressor capacity. Accordingly, the energy derived from the engine of the vehicle is effectively used for driving the vehicle. However, the pressure in the crank chamber may be increased to an excessively high value and maintained at that value until the crank and suction chambers are again linked, resulting in damage to the internal components of the compressor.
In order to address the above described defect, a safety valve device is proposed in Japanese Utility Model Application Publication No. 62-72473. This safety valve device includes a ball member and a coil spring elastically supporting the ball member and is disposed in a third communication path which links one portion of the first communication path upstream of the suction pressure sensing device to another portion of the first communication path downstream of the suction pressure sensing device. The safety valve device opens and closes the third communication path in response to changes in the pressure differential between the crank and suction chambers. The third communication path is opened when the pressure differential between the crank and suction chambers exceeds a predetermined value which is set to avoid damage to the internal components of the compressor. Therefore, when communication between the crank and suction chambers is blocked while communication between the crank and discharge chambers is opened during operation of the variable displacement mechanism (which may cause an abnormal rise in crank chamber pressure by conducting refrigerant gas from the discharge chamber to the crank chamber), the third communication path is opened to forcibly and quickly reduce the crank chamber pressure and thereby prevent an abnormal pressure differential between the crank and suction chambers. As a result, excessive friction between the internal components of the compressor caused by the abnormal differential between the crank and suction chambers can be prevented.
In the latter variable displacement mechanism, the third communication path is separate from the first and second communication paths so that the process of forming the third communication path and the process of disposing the safety valve device in the third communication path are additional steps required during the manufacturing of the compressor. Accordingly, the manufacturing process of the compressor is greatly complicated.